New Encore method for adding multiple entries at once

[shmolf]

Purpose: I dislike having to call addEntry multiple times, and would prefer the cleaner look of a single hash.

Example Usage:

Encore.addEntries({
  main: './path/to/main.js',
  secondary: './path/to/another/file.js',
});

This supports validation that any keys provided in the Object literal do not yet exist as an Entrypoint or Style Entrypoint.

Tests have also been updated.

---

Another new method (validateNameIsNewEntry) was also added to extract and consolidate logic related to checking for duplicate entrypoint names.

Tests for this other function have also been added.

[weaverryan]

This is really, really nicely done! I don't know if personally need the feature, but it's subjective, and you implemented this beautifully! 👍